The present invention relates to a device for providing constant current, and more particularly, to a device having temperature compensation for providing a substantially constant current through utilizing a compensating unit with a positive temperature coefficient.
In many analog integrated circuits, a constant voltage or a constant current source is needed for the operation of the whole circuit. The constant voltage source or the constant current source, therefore, plays an important role and can deeply affect the system performance. Usually, in a constant current source circuit, there is a band-gap block used as a temperature-independent voltage generating circuit to supply a constant voltage which is transformed into current by utilizing a resistive load. Considering no other factors, the induced current is a constant current. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a structure of a related art constant current source 100. As shown in FIG. 1, the constant current source 100 includes a band-gap block 110 for providing a constant voltage VBP; an operational amplifier 120, coupled to the band-gap block 110, for receiving the constant voltage VBP and a load voltage Vload as a negative feedback to hold the load voltage Vload equal to the constant voltage VBP by outputting an output voltage Vout; a current source 130, coupled to the operational amplifier 120, for receiving the output voltage Vout to provide the load voltage Vload and to provide a necessary amount of current to be drained; and a resistor 140, coupled to the load voltage Vload, for transforming the constant load voltage Vload into a substantially constant current Iconst drained from the above current source 130.
However, in practice, a resistive value (resistance) of the resistor 140 varies slightly when the resistor 140 experiences a temperature variation. This causes a magnitude of the current Iconst to fluctuate due to a temperature variation and thus makes the constant current source 100 fail to maintain a constant current as desired.
In a related art technique, the above-mentioned resistor 140 is replaced by a compensating load that is composed of a resistor and an NMOS transistor that is operated in the saturation region. Please refer to FIG. 2. FIG. 2 is a schematic diagram of a compensating load 200 according to the related art. The compensating load 200 includes a resistor 210 and an NMOS transistor 220. The resistor 210 possesses a positive temperature coefficient such that as the ambient temperature increases, the resistive value (resistance) of the resistor 210 increases accordingly, leading to a current flowing through the resistor 210 to decrease. However, since the threshold voltage of the NMOS transistor 220 also decreases when the ambient temperature increases, there will be a larger voltage drop across the resistor 210 compared to an original voltage drop across the resistor 210 before the ambient temperature changes. This reduces the current flowing through the resistor 210, but increases a voltage drop across the resistor 210. Thus this compensating load 200 is able to compensate the current for the temperature variation. Nevertheless, the related art technique is limited to compensating for a resistor with positive temperature coefficient. Very often, rather than a resistor with a positive temperature coefficient, one needs to compensate for a resistor with a negative temperature coefficient. For example, in VLSI, a resistive device can be composed of poly silicon and may possess negative temperature coefficient for a resistive value corresponding to the resistive device. Therefore, in order to stabilize the current flowing through a resistor with a negative temperature coefficient, it is desired to provide a compensating mechanism satisfying this constant current requirement.